Hitless (error-free) switching is required mainly in the case of fiber-maintenance/construction. When telecommunication cables are relocated in an existing digital network, the information that was transported on these cables must be redirected on an alternative path to avoid interruption of services. In such cases, the network operator must redirect manually the traffic on the affected path over an alternative path, that most probably has a different length and a different number of network elements (NEs).
In case of protected networks, it is possible to use the protection fiber to accommodate the traffic affected by the relocation/maintenance operation.
However, in general, hitless manual switching operation requires reconfiguring the network for locating an alternative path to accommodate the disrupted traffic, which is not an easy task.
As there is a difference in the length of the permanent and alternative paths and in the number of NEs in each of these paths, the signals travelling along these two paths generally experience a differential delay, which must be aligned at the receiver site in order to obtain hitless switching.
There is a need to achieve cross-connection of the paths without disrupting the services, whereby eliminating the need for network operations associated with switching traffic from one path to another.
There is also a need to provide a communication network with hitless switching capabilities for traffic at small physical granularity, for flexibility and cost purposes.